Industrial fuel and/or flammable liquid storage tanks frequently have a roof creating a space above the liquid, usually a fixed conically- or geodesically-shaped roof welded to the top of the tank. Such tanks may have a double roof, including an internal floating roof, called a floater, designed to float on top of the fuel/liquid with seals for sealing against the inside tank wall. The fixed cone or geodesic top roof is typically attached by welding. A roof system comprised of either a single fixed top portion or of two portions, a fixed top and a floater, creates and defines a space or cavity between either the surface of the fuel/liquid and/or the floater below and the top roof above.
Fire in an enclosed or fixed roof industrial storage tank can present unique problems. Industrial storage tanks are tanks with a diameter of a 60 feet or greater. Tank walls are typically 50 feet high, and usually 45 feet or higher. The top of the tank wall just below the rim is usually characterized by a series of eyebrow vents around its circumference. The eyebrow vents serve to allow vapor to escape from the tank rather than collect. Collecting of vapor within the tank presents numerous hazards. Thus, vents are typically provided to vent to the atmosphere vapors that collect in the space or cavity between the fuel/liquid (or floater) and a top fixed roof. The usual vents are “eyebrow vents” comprising spaced rectangular openings around a top portion of the vertical tank wall, scooper vents provided at the top of the tank and/or roof vents comprising spaced openings around the periphery of the top roof.
When vapor from the liquid within the tank, such as gasoline or crude, catches fire typically the roof is not blown off. The fire manifests itself predominantly at a downwind set of vents. (Air is sucked in through upwind vents.) The liquid within the tank is vaporizing, but the space above the liquid level and below the fixed roof typically offers insufficient oxygen to sustain a full burn. (The vapor mixture in the cavity may partially combust in the cavity.) The full ignition and complete combustion occurs, however, as the vapor exits the vents and comes into contact with the oxygen available in the atmosphere.
In the event of a fire in a fuel or flammable liquid tank having a fixed top roof, it is industry standard procedure, regulated by the NFPA, to extinguish the fire (or at least to attempt to do so) by a foam attack. The attack comprises laying a foam blanket on the fuel/liquid surface typically by discharging foam into the space or cavity between a fixed top roof and the liquid surface and/or a floater. As discussed above, full combustion typically occurs only at the vents where the fuel/liquid vapors meet atmospheric air, and typically only at the downwind vents while air is sucked into the cavity at the upwind vents.
NFPA has guidelines for the rate of foam application and the duration of a foam attack, adjusted for different type fuels or flammable liquids, different foams and different tanks, in order to achieve extinguishment.
It has been discovered that after laying down an adequate foam blanket, adequate by regulatory standards in the covered tank situation, the fire may persist. Observation indicates that significant vapor is yet trapped in the space above the foam blanket and below the fixed roof, and a certain amount of rich incomplete combustion may be taking place on top of the foam blanket within the enclosed space. Furthermore, since foam has a 25% drain time of 2-8 minutes, foam blankets are not permanent. Foam blankets, sooner or later, can permit vapors to pass through, replenishing the vapor supply within the space above the blanket. If the cavity above the liquid level and below the fixed roof is significant, the burn time for just the vapor trapped in the space, without any replenishment, could be long, several hours, which is unacceptably long even if an adequate foam blanket permits no further vapor to pass through. Foam supplies are limited, which dictates that the fire be extinguished within a given period of time.
An experience extinguishing a blended fuel tank fire in Guatemala demonstrated that foam alone may not extinguish a difficult fuel or flammable liquid fire in a storage tank having a fixed top roof, even when foam is placed in the cavity in accordance with NFPA recommended procedures, rates and durations. This appears disturbingly true of the new blended fuels having a high-octane content. It is a disconcerting discovery. Foam alone may not extinguish the fire at all, and quite likely will not do so per current NFPA regulations or guidelines. The instant inventor has further experience of vapor passing through a foam blanket and creating vapor foam bubbles. The bubbles drain out of vents and fall down to the ground burning.
It has been further determined that any flammable liquid, including crude, may produce vapors in a cavity above the surface of a liquid and below a roof that can keep a fire burning (at least at certain vents if not also, to an extent, within the cavity) for a significant time period after establishing a foam blanket. This fire may persist after the establishment of an NFPA adequate foam blanket. If the liquid level in the tank is low enough, and/or if the cavity is sufficiently large, fire from cavity vapor can persist for hours after the establishment of an adequate foam blanket. During this period foam dries out and vapors can traverse the foam blanket, replenishing the vapor in the cavity. Foam supplies may not be adequate to maintain a sufficient fresh foam blanket for hours.
The instant invention teaches, therefore, an improved system designed to cost effectively extinguish not only a “difficult fire” in a tank with a fixed roof, or a roof that creates a space between the roof and the liquid, but also a fire of any flammable liquid. The improved system is designed to cost effectively extinguish a fire of a difficult to extinguish fuel or flammable liquid having a high-octane content as well as a fire of any flammable liquid. The invention teaches a staged and timed discharge of dry chemical into the space between the burning fuel/liquid and the roof. The timing of the staging of the discharge of the dry chemical is selected to follow a pertinent period of foam application. Dry chemical is a limited and rationed resource. Discharging the dry chemical too soon might be ineffective and, thus, waste the resource.
The discharging of the dry chemical can be effected by one of several means or techniques, using portable and/or fixed systems. (A “fixed system” is equipment put in place prior to a fire, fixed prior to an emergency, in anticipation of emergencies. In contrast, portable systems are brought to the locale of the emergency upon notice.) Vents provided to vent vapors that collect under a roof can be advantageously used as an entry means to discharge the dry chemical into the space above the fuel/liquid and below the roof. Both portable and fixed systems could utilize existing vents. Alternately, special ports for fixed foam systems can be utilized for a fixed dry chemical system; as well, special ports for a fixed dry chemical system can be created.
It is the inventor's experience and observation that dry chemicals, timely inserted into a space between burning fuel/liquid and the roof, after a substantial foam attack, chase remaining persistent, pernicious fire or vagrant flames in the cavity and serve to completely extinguish the fire. The movement of air into the cavity through upwind vents or openings, as discussed above, aids a discharged suspended dry chemical in chasing the flames. Foam alone is an inferior and more costly means, if not an inadequate means, to completely extinguish residual flames in such a tank. Furthermore, foam is expensive. The extra time required to secure extinguishment by foam, even if it can be achieved with a continued application of foam alone as compared to the instant invention, is unnecessarily costly.
The instant staged dry chemical methodology and apparatus for extinguishing a “fixed roof” (so to speak) tank fire may be implemented in various forms, including using portable apparatus and/or fixed systems. Fixed systems and/or special portable apparatus could be less risky for firefighters, and as such would be preferred over a portable embodiment requiring firefighters to climb the tank, walk over the roof and insert dry chemical through an existing or created vent or opportune opening with a hand held nozzle.
The term “difficult to extinguish fuel or flammable liquid” or “difficult fuel or flammable liquid fires” is used herein to refer to fluid fuels or flammable liquids that are, at least, in substantial part, low-surface tension fuels/liquids and/or high-vapor pressure fuels/liquids and/or octane-boosted fuels/liquids and/or oxygenated fuels/liquids. The implied comparison in these instances would be recognized by one of skill in the art to be with the historic straight chain fuels or flammable liquids of the mid-20th century.
The instant invention, however, has been determined to also have significant application with the historic straight chain fuels or flammable liquids of the mid-twentieth century.
It should be understood that although a tank may be designed with, and originally exist with, a particular roof system, the initiation of a fire or hazard may have altered or destroyed part or all of the original roof system. Thus, the characterization of a storage tank may have to be reassessed. Original floating roofs, or floating roof portions, may have tilted or partially sunk or totally sunk. Seals may have been destroyed, in whole or in part. Fixed roofs may have been blown awry, or may have been partially dislodged or tilted, or at least their connections, such as a welded connection with a tank wall, may have been partially or totally destroyed. The instant invention relates to a tank that, at the time of the fire, still has at least a significant roof portion creating a substantially enclosed space above the fuel/liquid and below the roof. That is, the invention relates to situations where a difficult fuel or flammable liquid is on fire and there is at least a significant roof portion above the fuel/liquid surface, defining a substantially enclosed space or cavity there between. Although welds may be blown off from an original fixed roof portion, and hatches and vents may be blown apart, the invention applies if there remains a significant space or cavity between a burning fuel/liquid and a roof portion. Note again: the fuel/liquid may be burning only where it secures sufficient oxygen, such as where fuel vapors meet the atmosphere at vents or other open portions. Partial combustion may be taking place in the cavity.